The TARDIS arrives in 2nd century Scotland to solve the mystery of the
vanished Ninth Roman Legion. The travellers find the soldiers massacred,
their bodies in a state of decay akin to decades of sunlight
deprivation. The Doctor and Nardole are captured by the Picts who were
fighting the Roman incursion, while Bill is rescued from an alien
predator by the survivors of the Lost Legion. An interdimensional
portal, hidden within a stone cairn, holds the secret to defeating the
monster -- but conflict between the Picts and the Romans threatens to
doom the world.